1. Field of the Invention
The present invention relates to a charging apparatus and an image forming apparatus having the charging apparatus.
2. Description of the Related Art
In an image forming apparatus which employs an electrophotographic method, a charging apparatus of a corona discharge type is used for its charging section which uniformly charges a photoreceptor, namely an image bearing member which bears an electrostatic latent image thereon, transfer section which transfers a toner image formed on the photoreceptor onto a recording sheet, which is a material subjected to transfer and a recording medium as well, via a transfer belt which is a member subjected to transfer and an intermediate transfer member as well, separating section which strips the recording sheet kept in electrostatic contact with the photoreceptor, and so forth. Such a corona discharge-type charging apparatus is disclosed in Japanese Unexamined Patent Publication JP-A 6-11946 (1994). The charging apparatus disclosed in JP-A 6-11946 is composed of a shield case having an opening formed face to face with a to-be-charged body such as a photoreceptor and a transfer belt, and a discharge electrode installed inside the shield case in a stretched manner, the discharge surface of which has a linear shape, a serrated shape, or an acicular shape. The charging apparatus disclosed in JP-A 6-11946 is built as a so-called corotron charger and a so-called scorotron charger as well. In the corotron charger, a high voltage is applied to the discharge electrode to cause corona discharger so that the to-be-charged body can be uniformly charged. In the scorotron charger, a grid electrode is disposed between the discharge electrode and the to-be-charged body, and a voltage of a desired level is applied to the grid electrode so as for the to-be-charged body to be uniformly charged.
FIG. 10 is a view for explaining a charging mechanism associated with a corona discharge-type charging apparatus. Through the application of a high voltage to a region between a discharge electrode 71 having a small radius of curvature and a grid electrode 72, a non-uniform electric field is generated between the two electrodes. In consequence, a local ionization effect is produced in the vicinity of the discharge electrode 71 by an intense electric field, whereupon electrons are discharged in a direction D facing a to-be-charged body 11 (a discharge phenomenon induced by an avalanche of electrons). In this way, the to-be-charged body 11 is discharged. Note that the grid electrode 72 is disposed to control the quantity of electrons that travel toward the to-be-charged body 11. The grid electrode 72 is also subjected to electron discharge.
Moreover, the corona discharge-type charging apparatus described just above is also utilized as a before-transfer charging apparatus in which a toner image formed on the to-be-charged body 11 is charged before being transferred onto a material subjected to transfer. Such a corona discharge-type before-transfer charging apparatus is disclosed in Japanese Unexamined Patent Publications JP-A 10-274892 (1998) and JP-A 2004-69860, for example. According to the before-transfer charging apparatuses disclosed in JP-A 10-274892 and JP-A 2004-69860, in a case where a toner image formed on the to-be-charged body 11 exhibits variation in charging amount, the amount of charge on the toner image is made uniform before the toner image is transferred onto the material subjected to transfer. This makes it possible to suppress a decline in transfer margin at the time of transferring the toner image onto the material subjected to transfer, and thereby transfer the toner image onto the material subjected to transfer with stability.
However, the corona discharge-type charging apparatuses disclosed in JP-A 6-11946, JP-A 10-274892, and JP-A 2004-69860 described supra present a plurality of problems. The first problem resides in a space for placing the charging apparatus. The corona discharge-type charging apparatus necessitates not only the discharge electrode 71 but also the shield case, the grid electrode 72, and so forth. Furthermore, there is a need to secure a relatively large spacing between the discharge electrode 71 and the to-be-charged body 11 (approximately 10 mm, for instance). It will thus be necessary to provide a wider space to place the charging apparatus. In an image forming apparatus, in the vicinity of the charging apparatus are arranged a photoreceptor, a developing section which forms a toner image on the photoreceptor by supplying toner to an electrostatic latent image formed on the photoreceptor, a primary transfer section which transfers the toner image formed on the photoreceptor onto a transfer belt, a secondary transfer section which transfers the toner image formed on the transfer belt onto a recording sheet, and so forth. For that, there is not sufficient space for the placement of the charging apparatus. This makes difficult the layout of the corona discharge-type charging apparatus which occupies a relatively large space.
The second problem resides in a discharge product which is formed when the to-be-charged body 11 is charged by the charging apparatus. In the corona discharge-type charging apparatus, as shown in FIG. 10, discharge products such as ozone (O3) and oxides of nitrogen (NOx) are produced in large quantity. To be specific, with the energy released in accompaniment with the discharge of electrons from the charging apparatus, molecular nitrogen (N2) present in the air is dissociated into nitrogen atom (N), and the nitrogen atom is combined with molecular oxygen (O2) to form oxides of nitrogen (nitrogen dioxide: NO2). Similarly, molecular oxygen (O2) present in the air is dissociated into oxygen atom (O), and the oxygen atom is combined with molecular oxygen (O2) to form ozone (O3). If ozone is produced in large quantity in that way, various problems will arise such as occurrence of ozone odor, detrimental effects on the human body, and quality degradation in components induced by significant oxidative power. On the other hand, if oxides of nitrogen are produced, a defective image will be caused, because the oxides of nitrogen are adhered to the photoreceptor as ammonium salt (ammonium nitrate). In particular, in a case where an organic photoreceptor (OPC) is used as the photoreceptor, image imperfection such as a white patch or friar and image deletion caused by ozone and nitrogen oxides tends to occur.
The third problem resides in a corona wind which is generated when the to-be-charged body 11 is charged by the charging apparatus. The corona wind is generated in a direction from the discharge electrode 71 to the to-be-charged body 11 with the flow of electrons released by corona discharge. In the case of using the corona discharge-type charging apparatus as a before-transfer charging apparatus, a toner image formed on the to-be-charged body 11 will be distorted due to the corona wind.
As a charging apparatus which is capable of reducing discharge product generation, there has been proposed a charging apparatus of a contact charging type in which charging is effected by bringing a conductive roller or a conductive brush into contact with a to-be-charged body. In this contact charging-type charging apparatus, however, since the to-be-charged body is charged through the contact with the conductive roller or conductive brush, it is difficult to achieve the charging without distorting a toner image formed on the to-be-charged body. Therefore, the use of the contact charging-type charging apparatus as a before-transfer charging apparatus will be inappropriate.
Moreover, in Japanese Unexamined Patent Publication JP-A 8-160711 (1996) is disclosed a corona discharge-type charging apparatus which is capable of reducing discharge product generation. The charging apparatus disclosed in JP-A 8-160711 is composed of: a plurality of discharge electrodes arranged at a substantially uniform pitch in a predetermined axial direction; a high-voltage power supply which applies, to the discharge electrode, a voltage which is greater than or equal to a predetermined discharge inception voltage; a resistor element placed between an output electrode of the high-voltage power supply and the discharge electrode; a grid electrode placed at a position between the discharge electrode and a to-be-charged body in the proximity of the discharge electrode; and a grid power supply which applies a predetermined grid voltage to the grid electrode. The gap between the discharge electrode and the grid electrode is set at or below 4 mm. In this way, by adjusting the gap between the discharge electrode and the grid electrode to be small, it is possible to decrease discharge current and thereby reduce discharge product generation.
However, in the charging apparatus disclosed in JP-A 8-160711, ozone of approximately 0.3 ppm is evolved. That is, the effect of reducing discharge product generation is not good enough. Furthermore, in the charging apparatus disclosed in JP-A 8-160711, the gap between the discharge electrode and the grid electrode is so small that discharge products, as well as foreign matters such as toner and powdery paper originating from a recording sheet which is a material subjected to transfer, are prone to adhere to the discharge electrode. The removal (cleaning) of such a foreign matter adhered to the discharge electrode is difficult to achieve, because the discharge surface of the discharge electrode in the corona discharge system is given a complicated configuration, such as an acicular shape. In addition, the front end of the discharge electrode is susceptible to abrasion and quality degradation due to discharge energy, which results in lack of stability in the discharging effected by the discharge electrode. Furthermore, in the charging apparatus disclosed in JP-A 8-160711, since the to-be-charged body is arranged in a short distance away from the discharge electrode, it is likely that variation in charging will arise in a lengthwise direction due to the pitch of arrangement of a plurality of discharge electrodes (the axial direction in which are arranged a plurality of discharge electrodes). Although it may be possible to reduce the pitch of arrangement of the discharge electrodes to eliminate the charging variation, the reduction of pitch leads to an increased number of discharge electrodes and thus to increased manufacturing costs.
In Japanese Unexamined Patent Publication JP-A 2003-249327 is disclosed a charging apparatus of a creeping discharge type. In the creeping discharge-type charging apparatus, an induction electrode and a discharge electrode having a peaked plate-shaped outer periphery are arranged face to face with each other, with a dielectric body lying therebetween. A to-be-charged body is placed opposite from the induction electrode so as to face the discharge electrode. In the creeping discharge-type charging apparatus, a pulse waveform voltage is applied between the two electrodes thereby to cause emission of ions. The to-be-charged body is charged by the resultant ions.
The creeping discharge-type charging apparatus does not necessitate a shield case, a grid electrode, and so forth that are provided in the corona discharge-type charging apparatus. Therefore, a space for placing the charging apparatus can be made relatively small. Moreover, in the creeping discharge-type charging apparatus, the discharge electrode is formed in the shape of a plate, and its discharge surface is made flat. Accordingly, even if a foreign matter is adhered to the discharge electrode, the foreign matter can be cleaned off with ease. Further, in the creeping discharge-type charging apparatus, no corona wind is generated. This is because discharging is effected between the discharge electrode and the induction electrode. It thus never occurs that a toner image formed on the to-be-charged body is distorted by the corona wind.
In the creeping discharge-type charging apparatus disclosed in JP-A 2003-249327, however, the ions generated through the application of a pulse waveform voltage between the discharge electrode and the induction electrode remain in the vicinity of the discharge electrode without traveling vigorously toward the to-be-charged body. In this case, there arises variation in the flow of ions during the interval when the ions remaining in the discharge electrode are being moved to the to-be-charged body. This makes it difficult for the to-be-charged body to be charged uniformly. Furthermore, since the ions generated through the application of a pulse waveform voltage remain in the vicinity of the discharge electrode, it follows that, with respect to the quantity of ions generated, the ions which can be utilized to effect charging on the to-be-charged body are few in quantity. This leads to poor ion-use efficiency. In order to increase the quantity of ions which travel toward the to-be-charged body, there is a need to increase the applied voltage or frequency of the pulse waveform voltage. However, the increase of the applied voltage or frequency requires a measurable amount of electric power and also causes a decrease in the service lifespan of the discharge electrode. In addition to that, the generation amount of discharge products such as ozone will be increased.